Aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are well known engineering resins due to their well-balanced physical properties, such as excellent heat-resistance, mechanical strength and gas impermeability. As such, aromatic polyesters are employed in a variety of end-use applications. However, since aromatic polyesters have glass transition (Tg) temperatures well in excess of room temperature, they typically cannot be used when high flexibility at low temperatures is a requirement of the end-use application. Another problem experienced by aromatic polyester resins is that, although the resins inherently exhibit excellent gas impermeability, this inherent property cannot effectively be utilized in film-form due to the poor heat sealing properties of films made of aromatic polyesters. It is towards solving these problems typically associated with aromatic polyesters that the present invention is directed.
Broadly, the present invention involves incorporating a specified comonomer unit in an aromatic polyester chain so as to form a copolyester having excellent color hue, high hydrolysis resistance and increased flexibility which is essentially unaffected even at low temperatures. The copolyester resins of the present invention may be formed into a film which exhibits excellent heat sealing properties.
The present invention more particularly relates to a copolyester resin produced by the polycondensation of (a) terephthalic acid or a dialkyl terephthalate, (b) at least one alkylene glycol having 2 to 8 carbon atoms and (c) a dimer diol mainly comprised of a compound of the following formula (1): ##STR2## wherein R.sub.1 through R.sub.4 are each unsaturated linear organic groups such that the total number of carbon atoms for R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is between 22 to 34, and specifically, R.sub.1 and R.sub.2, which may be the same or different, each represent an alkyl group, and R.sub.3 and R.sub.4 which may be the same or different, each represent an alkylene group.
The copolyester resins of the present invention may be employed as a base resin to form suitable compositions depending upon the desired end-use application. Preferably, the copolyester resin according to the present invention will contain at least inorganic and/or organic filler material.
Further aspects and advantages of this invention will become more clear from the discussion which follows.